Conventional CW FM TR radars are generally limited to applications wherein a return signal is adapted to contain information regarding only one target. In other words, CW FM TR radars have not been designed for applications wherein a return signal may contain different returns derived from multiple targets and/or via multipath, i.e., transmission from a single target to a receiver via a direct path, as well as via reflected paths. This limitation arises because the multiple, different returns arrive at the receiver at different times, with different frequency and amplitude components to produce a composite return signal simultaneously containing many different components that have not been easily separated using conventional prior art techniques; see Introduction To Radar Systems, written by M. I. Skolnik, published by McGraw-Hill in 1962, chapter 3, pages 89 and 90.
In the typical prior art CW FM TR radar, the return signal is demodulated to derive a replica of the modulation imposed on the return signal. The demodulated signal is a function of the amplitude and carrier and modulation phases of the several different returns that form the composite return signal. Unless one of the individual returns has an amplitude considerably in excess of all of the remaining returns (at least an order of magnitude greater), the demodulated signal is likely to have a phase component that cannot be associated with any particular return. Hence, a phase comparison of the detected, demodulated signal with the original modulation for a transmitted signal may not provide any significant information regarding any of the returns. A theoretical basis for the inaccuracies inherent in the prior art CW FM TR radars using a low modulation index is provided in my Doctor of Philosophy thesis entitled "Multiple Target Resolution in a CW-FM Tone Ranging System" submitted to the Department of Electrical Engineering, University of Cincinnati, in July 1975.
While CW FM TR radars employing a high modulation index have been proposed in order to reduce the problems associated with a return signal being responsive to multiple returns, these systems have certain theoretical and practical disadvantages. In particular, in the proposed radar no consideration is given to the average range error when there is relative movement between the different targets, in which case there is a variation in the phases of the carrier frequency components on the separate returns forming the composite return. Of course, a wide band FM system requires a wide band signal structure and the associated difficulties in designing a highly sensitive, wide band receiver. A low modulation index radar, on the other hand, has great appeal because of the availability of efficient carrier loop implementations.